1. Field of the Invention
The present invention relates to an antenna to be applied to a telecommunication system employing an electric wave comprising a particular polarization component, and more particularly, to an antenna and an electric device having the same, by which transmission and reception with improved electric wave efficiency for two frequency bands of this telecommunication system.
2. Related Art
Currently, there are a lot of telecommunication systems employing the electric wave comprising a particular polarization component (vertical polarization or horizontal polarization) such as mobile communication (e.g. mobile phone) and ground wave television broadcasting. Particularly, in the mobile communication, the electric wave comprising the vertical polarization is often used in view of influence of obstacles, communication distance and the like. Further, in accordance with increase in capacity of communication data amount, speedup of the communication, diversification of the communication system and the like, there is a tendency of using two or more different frequency bands even in the case that the communication form is analogous or similar for plural terminals such as the mobile phone communication.
When the antenna to be employed for the telecommunication system using the electric wave comprising a limited polarization component supports only one frequency band, its structure per se is often designed based on an element size which is not less than ¼ times of a wavelength of the electric wave of the frequency band used in the telecommunication system. Further, when a single antenna supports the different two frequency bands, its antenna is designed based on the element size which is not less than ¼ of the wavelength of each frequency band, more particularly, not less than ¼ of the wavelength of the electric wave of the wavelength at a low frequency band side. In addition, the antenna is designed for using a high-order harmonic wave component of the low frequency band side in some cases. However, when this element size cannot be held by the reason of e.g. downsizing of the electric device carrying the antenna, it is extremely difficult to satisfy enough transmission and reception characteristics unless a complex structure using together a wavelength-shortening effect of a dielectric material and an electric circuit element and the like.
So as to solve the above problem, Japanese Patent Laid-Open No. 8-23224 (JP-A 8-23224) discloses a representative example of antennas in which a transmission and reception characteristic of the electric wave comprising a particular polarized wave is improved by a simpler structure. The antenna as disclosed in JP-A 8-23224 comprises a structure corresponding to one using frequency band. Since a technique of feeding an electric power to two slot parts is realized by a branched feeding line. It is sufficiently assumed that the single body can apply to the different two frequency bands according to the structure of the slot part. In other words, such a structure comprises two slot parts which mainly transmit and receive a vertical polarization wave and are disposed horizontally with respect to the earth and opposed to each other, and lateral lengths of the two slot parts are different from each other to correspond to the different two frequency bands. According to this structure, it is contemplated that it is possible to realize an antenna by which the transmission and reception of the electric wave comprising the particular polarization in the different two frequency bands with high efficiency can be achieved in the single device with the simpler structure. However, for this case, it is assumed that an enough size of a conductor part provided between the both slots (i.e. an interval between the slots) is required such that each of the two slot parts operates usefully for the different two frequency bands. Therefore, the dimensions of the antenna is required to be equal to or greater than a sum of the lengths of the two slot parts having the different lateral lengths and a length of the conductor part provided between the two slot parts. Accordingly, it is difficult to downsize the antenna. Further, in accordance with the increase in the antenna size, it is necessary to consider a length of each feeding line extending from a branching part of the feeding line to each slot part and an installation location of the feeding line. As a result, the structure of the antenna is complicated, and it is very likely that design of the device becomes difficult finally.
As described above, it is difficult to realize an antenna, by which the transmission and reception of the electric wave comprising the particular polarization component in the different two frequency bands with high efficiency can be achieved in the single device by a small sized antenna with the simpler structure.